Soil compaction machines usually have vibrating plates which move at a certain frequency to compact soil material by vertical impacts. The impacts are produced by vertically directed vibrations of the vibrating plate created by an opposing pair of unbalanced mass shafts. The unbalanced mass rotates in synchronization but in the opposite direction of rotation, resulting in a centrifugal force in one direction of vibration. By phase displacement of the unbalanced masses, it is thus possible to adjust a desired direction of vibration, which may deviate from the vertical direction of vibration, so that in addition to the vertical component of vibration, a horizontal component of vibration is also generated. The horizontal component of vibration produces a forward or reverse movement of the soil compaction machine.
The publication DE 101 21 383 C2, for example, discloses a control unit for an unbalanced mass adjusting device with a vibration generator of a soil compaction machine. An adjusting cylinder may be used here for adjusting phase angles of unbalanced masses in the vibration generator with the help of a reference piston which is connected to a switching valve. The switching valve serves to control an oil flow from a hydraulic oil source or an oil drain to an oil return flow and from the reference piston. The user of the soil compaction machine can adjust the switching valve electrically or mechanically between two positions, thereby permitting movement of the adjusting cylinder in both directions in order to select the forward or reverse movement of the soil compaction machine. One disadvantage of such units that control the direction of travel is that measured value pickups are required.
The publication DE 199 12 813 C1 also discloses a unit for controlling the direction of travel for a soil compaction machine. It includes a movable travel lever, the positions of which are detected by a sensor which then triggers a switching valve. The switching valve controls an adjusting cylinder in a vibration-generating device so that the cylinder is displaced between a starting position and an end position. Depending on the position of the adjusting cylinder, the soil compaction device travels forward or in reverse. With this soil compaction machine it is also possible to adjust only one forward or reverse movement, which is determined by the resulting centrifugal force in the end positions of the adjusting cylinder. However, it is not possible to adjust the speed of travel in a controlled manner.